Fuel cells use the chemical reaction of a fuel with oxygen to form water, in order to generate electrical energy. For this purpose, fuel cells contain, as the core component, the so-called membrane-electrode assembly (MEA), which is a composite made of an ion-conductive, in particular proton-conductive membrane and an electrode (anode and cathode) situated on either side of the membrane. In addition, gas diffusion layers (GDL) may be situated on both sides of the membrane-electrode assembly, on the sides of the electrodes facing away from the membrane. In general, the fuel cell is formed by a plurality of MEAs situated in a stack, the electrical powers of which are added together. During operation of the fuel cell, the fuel, in particular hydrogen H2 or a hydrogen-containing gas mixture, is supplied to the anode, where an electrochemical oxidation of H2 to form H+ with the emission of electrons takes place. A (water-bound or water-free) transport of the protons H+ from the anode chamber into the cathode chamber takes place via the electrolyte or the membrane, which separates the reaction chambers from one another in a gas-tight manner and electrically insulates them. The electrons provided at the anode are conducted via an electrical line to the cathode. Oxygen or an oxygen-containing gas mixture is supplied to the cathode, so that a reduction of O2 to form O2− with absorption of the electrons takes place. At the same time, these oxygen anions react in the cathode chamber with the protons transported via the membrane to form water. Due to the direct conversion of chemical energy into electrical energy, fuel cells achieve an improved efficiency in relation to other electricity generators because of the avoidance of the Carnot factor.
The fuel cell is formed by a plurality of membrane-electrode assemblies situated in the stack, so that reference is also made to a fuel cell stack. A bipolar plate, which ensures a supply of the individual cells with the operating media, i.e., the reactants and a coolant liquid, is situated between each two membrane-electrode assemblies. In addition, the bipolar plates ensure an electrically conductive contact to the membrane-electrode assemblies. Furthermore, they ensure a sealed separation between anode chamber and cathode chamber.
The bipolar plates are usually constructed from two profiled electrode plates, which have a structure in the form of a vertical profile situated on both sides of the plates. More or less discrete channels, which are designed to guide the operating media, result due to this profile on both sides of the plates. The operating media are in turn separated from one another by the plates, so that the coolant is guided in the interior of the plate, while the reactant gases are guided outside. The channels of the reactant gases are delimited, on the one hand, by the particular plate and, on the other hand, by a gas diffusion layer.
The bipolar plates may have different structures for distributing the reactants (fuel and oxidants) over the membrane surface. For this purpose, for example, the channels described in U.S. Pat. No. 4,988,583 are known, which are guided in a meandering form over the plate. They ensure a good uniform distribution of the operating media at both low and high flow rates. However, meandering structures have the disadvantage that at high flow rates, they cause large pressure losses from a first distributor area (inlet) to a second distributor area (outlet). The necessity thus arises of supplying the operating media at high pressure, which results in energy losses for the overall system.
In contrast, if the operating media are guided by a plurality of linear channels from the first distributor area to the second distributor area, irregular distribution occurs regularly at high flow rates as a result of the lack of a possibility for lateral distribution of the flow. However, they have a substantially lower pressure loss over the length of the fuel cell than the meandering structures.
In addition, pressure differences between adjacent flow channels are of substantial importance for the design of a fuel cell. Serpentine flow channels as described in WO 2005/112163 A2 are known for homogenizing the pressures both between distributor areas and also between the channels. Serpentine flows generally have an odd number of legs, which extend in the form of hairpin turns over the distributor areas or the bipolar plate. Different widths, depths, and lengths of the flow channels are used in this case to change the hydraulic cross section of the channels locally in such a way that targeted pressure differences arise, which accelerate the operating media within the channels or even to promote a transverse flow over the MEA.
DE 103 94 052 T5 describes, as a refinement, a flow field of a PEM fuel cell, which includes flow channels having branching overlap sections adjoining the distributor areas, to reduce the pressure differences between the distributor areas.
To intentionally achieve a transverse flow of the operating means across the area of the bipolar plate, DE 101 63 631 A1 provides a special arrangement of webs on the surface of the bipolar plate, multiple webs interrupted by outlets being situated in succession within a row.
In a comparable approach, DE 10 2005 057 045 A1 describes a structure of a bipolar plate, the typical channel structure being interrupted in the distributor area in favor of a structure which is made of minimal support points.
In addition, the goal is pursued in the development of fuel cells of further reducing the height of a bipolar plate and therefore the height of the fuel cell stack. The problem results from the reduction of the height of the bipolar plates that the overall height also has to be reduced in inflow areas of the fluids from the ports situated on the edge to the actual fluid channels in order to be able to reduce the height of the entire bipolar plate. On the one hand, the inflow area is to occupy a preferably small installation space, on the other hand, it is to be sufficiently large so as to ensure a uniform distribution of the fluids. This is problematic in particular for hollow embossed bipolar plates, for example, made of thin metal plates, because the fluids intersect in the inflow area. This means that the height of the inflow area has to be reduced still further.
A bipolar plate is known from unexamined published application DE 100 150360 A1, in which an intersection of fluids in thin bipolar plates is implemented, a cooling fluid being guided transversely via a perpendicularly situated structure of the gas guiding channels. The channel depth may be reduced on the anode and cathode sides in the areas in which the cooling fluid intersects.